a) Field of the Invention
The present invention relates to thrusters used in boats, and more particularly to stern thrusters which are commonly positioned at the transom of the boat. The present invention is particularly adapted for use in stern thrusters where the draft of the boat (i.e., the depth to which the boat floats in the water) is rather small.
b) Background Art
When a ship is traveling forwardly in the water, the rudder is used to exert a lateral force so as to cause the boat to turn one way or the other. However, when the boat is in “tight” locations with little or no forward travel, maneuvering the boat can be rather difficult.
Accordingly, there are various other means used or proposed for use to improve the maneuverability of the boat in a limited operating area (e.g. in docking and/or other maneuvers), and one of these is to provide thrusters, such as stern thrusters.
The main purpose of the stern thruster is to move the stern of the vessel to port or starboard when there is little or no forward or reverse motion of the vessel. In general, there are three types of stern thrusters which are currently on the market. One of these is the water-jet thruster which discharges jets of water to produce the thrust. These are somewhat expensive and less efficient in comparison with other types of thrusters, and their primary use currently is on fire-boats where there is room for a large engine powering an onboard water pump.
Another type of thruster is the use of a propeller or propellers connected to a hydraulic motor permanently fastened to the vessels' transom. To the best knowledge of the applicants, there is only one such thruster being currently marketed for yachts.
A more common currently used stern thruster is a tunnel-propeller thruster where there is a laterally aligned housing in the form of a cylindrical duct or tunnel positioned at the transom below the water, with one or two propellers positioned in the duct or tunnel.
The tunnel thruster needs to be positioned far enough below the water surface to prevent air being sucked into the tunnel passageway along with the water traveling through the propellers, since this can cause a substantial loss of thrust. Thus, it is generally recommended that the thruster be positioned in the water at least one tunnel diameter below the water line.
However, for smaller boats which have a rather shallow draft, a thruster permanently installed in the transom of the boat has in general been impractical. The dimensions of the thrust apparatus must be sufficiently large to be able to eject water at a volumetric rate sufficient to provide adequate thrust for maneuvering, and yet (as indicated above) be a sufficient distance below the surface of the water so that it will not lose thrust by sucking in ambient air. However, if the lower part of this thrust apparatus is too far down, portions of the thruster will be positioned in the water stream that passes under the hull of the boat, traveling at medium or full speed, thus causing substantial drag.
The result of this is that various stern thrusters have been available for larger boats which have deeper draft, but not for the relatively small boats that have rather shallow draft. One solution is to have a thruster that is vertically adjustable so that it can be lowered into the water when needed and raised upwardly a sufficient level when the boat is running at medium or high speed so as to be out of the water stream. However, for various reasons (quite possibly expense and/or complexity) to the best knowledge of the applicants, that design has not been widely accepted.
A search of the patent literature has disclosed a number of concepts relating to stern thrusters or the like, and these are listed below as follows.
U.S. Pat. No. 6,435,120 B2 (Duncan) shows a lateral thruster for a boat, where there are right and left thrusters each having its own housing with a thrusting propeller and a stator. Surrounding both of these right and left thrusters is a larger duct having open side ends and enclosing the area around the two thrusters, and also enclosing the area between the two thrusters. When one of the thrusters, for example the right thruster, is rotating to provide a thrust, the left thruster would be free-willing and water would flow in the surrounding duct and also through the left thruster toward the right thruster. Also, water from the right side of the surrounding housing would flow inwardly in an area surrounding the right thruster, and also be re-directed to go outwardly through the thruster.
U.S. Pat. No. 5,704,306 (Den Ouden) shows a “stern screw” where there is a lateral thruster in the form of a propeller that is positioned in a surrounding cylindrical housing which provides a “tunnel”. It is stated in column 3, line 4, that the tunnel tube lays at least one-half a tunnel tube diameter above the bottom 11 of the boat, and at least one tunnel tube diameter below the waterline. It is indicated that with such a placement of the tunnel tube, well below the waterline, it is found to benefit the propelling force of the stern screw.
U.S. Pat. No. 5,016,553 (Spencer) shows a boat having a thruster which is connected to the steering system of the boat, so that the direction of thrust is controlled from the boat's steering wheel linkage.
U.S. Pat. No. 4,832,642 (Thompson) shows a propulsion installation for a boat, where there are radial vanes which are positioned as a “paddle wheel” where these are positioned partly above the water with the vanes rotating down into the water to provide the thrust. There is shown a water intake at 16 for a hydraulic motor which is protected by a grill 17 to keep debris from coming into the inlet.
U.S. Pat. No. 4,402,674 (Roberts) shows a propulsion system which employs a water jet that ejects the water in a rearward direction. This patent discloses a water intake system which is arranged to prevent air being aspirated into the jet stream. This structure is located at the bottom of the hull and is designated as an “air aspiration prevention pan” which faces downwardly having side walls arranged in a general V-configuration with the apex of the V-configuration toward the forward end of the hull.
U.S. Pat. No. 4,208,978 (Eller) shows a lateral thruster having a cylindrical housing oriented transversely and a propeller thruster mechanism operating to provide the lateral thrust. There is a positioning mechanism which can lower the thruster into the water or pull it upwardly out of the water.